A colorimetric, label-free, and nonaggregation-based gold nanoparticles (Au NPs) probe has been developed for the detection of Pb(2+) in aqueous solution, based on the fact that Pb(2+) ions accelerate the leaching rate of Au NPs by thiosulfate (S(2)O(3)(2-)) and 2-mercaptoethanol (2-ME). Au NPs reacted with S(2)O(3)(2-) ions in solution to form Au(S(2)O(3))(2)(3-) complexes on the Au NP surfaces, leading to slight decreases in their surface plasmon resonance (SPR) absorption. Surface-assisted laser desorption/ionization time-of-flight ionization mass spectrometry (SALDI-TOF MS) data reveals the formation of Pb-Au alloys on the surfaces of the Au NPs in the presence of Pb(2+) ions and 2-ME. The formation of Pb-Au alloys accelerated the Au NPs rapidly dissolved into solution, leading to dramatic decreases in the SPR absorption. The 2-ME/S(2)O(3)(2-)-Au NP probe is highly sensitive (LOD = 0.5 nM) and selective (by at least 1000-fold over other metal ions) toward Pb(2+) ions, with a linear detection range (2.5 nM-10 muM) over nearly 4 orders of magnitude. The cost-effective probe allows rapid and simple determination of the concentrations of Pb(2+) ions in environmental samples (Montana soil and river), with results showing its great practicality for the detection of lead in real samples.

We have developed a simple method for the selective colorimetric detection of aqueous mercuric (Hg 2+ ), silver (Ag + ), and lead (Pb 2+ ) ions by using label-free gold nanoparticles (Au NPs) and alkanethiols. The degree of alkanethiol-induced aggregation of the Au NPs decreases in the order of 6-mercaptohexanol (6-MH) ∼ 4-mercaptobutanol (4-MB) > 11-mercaptoundecanol (11-MU) > 2-mercaptoethanol (2-ME). The specific and strong interactions of these alkanethiols with Au NPs and heavy metal ions enabled us to develop label-free assays for the sensitive and selective detection of Hg 2+ ions using the 4-MB/Au NPs probe, as well as Ag + and Pb 2+ ions using the 2-ME/Au NPs probe. The presence of strong Hg 2+ -S bonds alleviated the extent of 4-MB-induced aggregation of the Au NPs, resulting in a declining ratio of the extinction coefficients at 650 to 520 nm (Ex 650/520 , a measure of the molar ratio of the aggregated to the dispersed Au NPs) of the Au NP solution. In contrast, the presence of Ag + , Cu 2+ , and Pb 2+ ions led to a severe aggregation of the Au NPs, mediated by the deposition of these ions on the surfaces of the Au NPs in the 2-ME/Au NPs system. In the presence of masking agents [ethylenediaminetetraacetic acid (EDTA), Na 2 S], the 2-ME/Au NP-EDTA and 2-ME/Au NP-Na 2 S sensors permitted the selective detection of Ag + and Pb 2+ ions, respectively, at concentrations down to the nanomolar range. This cost-effective process also allowed the rapid and simple determination of the concentrations of heavy metal ions in real environmental samples (river water and Montana soil). These alkanethiol/Au NP-based sensor probes enabled us to detect three different heavy metal ions, and we feel confident that, because of the simplicity, rapidity, and cost-effectiveness of these analyses, such systems demonstrate great potential for the practical detection of heavy metal ions in real samples.

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